1. Field of the Invention
The present invention relates to an improved image forming apparatus having a serial paper feeding device and a single sheet feeding device which is capable of safely returning a series of paper at a printing area back to the serial paper feeding device without causing any paper jam when switching from a serial paper feeding mode to a single sheet feeding mode.
2. Description of the Related Art
Generally, an image forming apparatus such as an ink jet printer, etc. employs a single sheet feeding device for feeding sheets of paper one by one, and a serial paper feeding device for successively and continuously feeding the paper for printing. The single sheet and serial paper feeding devices are selectively mounted on the image forming apparatus depending on the need of the user.
Accordingly, for several purposes such as switching the serial paper feeding mode into the single sheet feeding mode, the image forming apparatus is provided with a function of retreating the serial paper from the printing area back to the serial paper feeding device, which is called the "parking" function.
During the parking operation, all the rotatable rollers inclusive of a tractor are reversely rotated. However, due to unbalanced movements of the respective parts, the paper is not safely returned to the tractor but is folded and separated from the tractor, causing the state called a "paper jam".
Referring to FIG. 1, in order to use a series of paper (P), a serial paper feed selection lever 1 is rotated in the direction indicated by a solid arrow. Accordingly, a cam shaft 2 is rotated to push a friction guide 3, and to lift a friction roller 4 for the conveyance of a single sheet.
In the single sheet feeding mode, the series of paper (P) is not affected by the movement of the feed roller 5 and the friction roller 4. Next, by rotating the tractor wheel 6 clockwise, the serial paper (P) secured between an eject roller 7 and a star eject wheel 8 is returned by the tractor wheel 6 which pulls the serial paper (P).
Further, in order to prevent a paper jam at the series of paper (P), the eject roller 7 is designed to have a slightly faster rotational speed than the tractor wheel 6. That is, by the speed difference, the paper (P) is smoothed. Here, since a slip occurs between the eject roller 7 and the eject wheel 8 to correspond to an extra conveyance distance of the paper (P), the series of paper (P) can be successively conveyed without having any problem even with the different speeds of the respective rollers.
In the serial paper feeding device, the tractor wheel 6 is movable leftward and rightward according to the width of the paper by a tractor shaft 9. By the structural need, there has to be a clearance (or space) between the tractor wheel 6 and the tractor shaft 9, and accordingly, a "backlash" is caused in the initial driving in which only the tractor shaft 9 rotates while the tractor wheel 6 does not.
Meanwhile, as shown in FIG. 2, since the eject roller 7, an eject shaft 10 for transmitting the driving force to the eject roller 7, and an eject gear 11 are securely press-fitted with each other. There is almost no backlash.
Accordingly, referring back to FIG. 1, when the series of paper (P) advancing to the forward direction (direction for the printing operation) is reversely retreated for the parking operation, the eject roller 7 reversely conveys the series of paper (P) even though a backlash is caused, i.e., even though the tractor wheel 6 is not rotated in its initial operation. Accordingly, since the tractor wheel 6 does not convey the series of paper as the eject roller 7 does, the series of paper (P) is folded as indicated by the dotted line, while the parking operation is kept on being performed. As a result, the series of paper is separated from the tractor wheel 6. By such a paper jam, there has been a reliability deterioration of the image forming apparatus.